In wireless communication, a multiple input multiple output (MIMO) technique, which improves a communication speed and reliability by using multiple antennas, is used in many systems including a cellular system and a wireless local area network (LAN) system. In the MIMO, a maximum likelihood estimation (ML) method is used as MIMO demodulation processing for estimating an original transmission signal from signals that are output from multiple antennas and mixed in a wireless propagation path. The ML method is known as an optimum demodulation method in the case where there is no a priori information regarding a received signal.
In the ML method, an original signal is estimated by comparing every possible received signal point candidates and a received signal. Therefore, in the case where many antennas are provided or a modulation multilevel number of a transmission signal is large, received signal point candidates are exponentially increased and a calculation amount becomes huge.
In this case, as a method for reducing a processing amount while suppressing deterioration of demodulation characteristics, a method using QR decomposition and M algorism (refer to NPL 1) and a sphere decoding (SD) method (refer to NPL 2) are proposed. In these methods for reducing a calculation amount, the calculation amount is reduced by narrowing received signal point candidates down to those to be used in processing by the ML method.
On the other hand, a technique called BICM-ID (bit interleaved coded modulation with iterative decoding) (refer to NPL 3) is proposed, and it has gained attention. In the BICM-ID, demodulation processing with respect to modulation and a decoding processing with respect to encoding are iteratively performed to realize excellent decoding characteristics. Characteristics of the BICM-ID are not individual characteristics of a demodulator and a decoder and are determined by matching therebetween. In the BICM-ID, convergence characteristics are analyzed by using extrinsic information transfer (EXIT) analysis proposed in NPL 4, and a demodulator and a decoder which realize excellent decoding characteristics can be designed.
NPL 5 discloses a method to provide the BICM-ID, in which a transmission rate loss and a processing amount of a decoder are reduced by encoding by a code on a basis of a repetition code considered to be the simplest code and by performing multilevel modulation using non gray mapping and extension mapping.